1. Field of the Invention
The present invention relates to a backlight unit for housing a lamp that is a light source of a display device with a backlight, and specifically relates to a backlight unit preferably used in a liquid crystal display device including a translucent liquid crystal display panel.
2. Description of the Related Art
A liquid crystal display device including a translucent liquid crystal display panel and the like, which are cited as an example of a flat-screen display device, generally have a backlight unit arranged behind the liquid crystal display panel. The backlight unit is a device including a tubular lamp such as a cold cathode tube as a light source, which controls the properties of light emitted from the tubular lamp and projects the light toward a rear side of the display panel. The projected light passes through the display panel, making an image displayed visible on a front side of the display panel.
FIG. 5 is an exploded perspective view schematically illustrating relevant parts of a structure of a generally conventional liquid crystal display device. A liquid crystal display device 30 includes a bezel 31, a display panel 32 and a backlight unit 33. The bezel 31 is a member that defines a frame of the display panel 32, and the display panel 32 is made by bonding two panels of glass so as to seal in a liquid crystal therebetween.
The backlight unit 33 includes a frame 34, optical sheets 35, tubular lamps 36, a reflector 37 and a backlight chassis 38. The frame 34 is shaped like a picture frame and secures the optical sheets 35 to the backlight chassis 38. The optical sheets 35 are provided for controlling the properties of light which is emitted from the tubular lamps 36 and enters the display panel 32. In this case, the tubular lamps 36 are U-shaped, and left-side ends thereof are inserted into electrode part holders 41 so as to be secured to the backlight chassis 38 at the left-end positions, as shown in a plan view of FIG. 6. The reflector 37 is laid under the tubular lamps 36, for reflecting the light emitted from the tubular lamps 36 toward the display panel 32. In order to improve reflectivity, projections 37a having a crest shape are provided on the reflector 37 so as to be located respectively between tube sections 36a of the tubular lamps 36.
The backlight chassis 38, substantially in the shape of a box, has a lamp housing portion 38a including a bottom portion 38b and side-wall portions 38c and 38d, and support surfaces 38e and 38f extending outward respectively from upper edges of the side-wall portions 38c and 38d. In the backlight chassis 38, the bottom portion 38b and the longer side-wall portions 38c are formed of a member 39 which is prepared by subjecting a metal plate material to plate metal processing, and the shorter side-wall portions 38d are formed of a member 40 which is molded of resin.
The tubular lamp 36 is secured to the lamp housing portion 38a of the backlight chassis 38 with the use of the above-mentioned electrode part holder 41, and also with the use of a member 44 which includes lamp clips 42 for holding the tube section 36a at an approximate midpoint thereof, and a sheet holding pin 43 for preventing the optical sheets 35 which are arranged above from bending downward to preclude luminance irregularity, as illustrated. The member 44 including the lamp clips 42 and the sheet holding pin 43 is secured to the backlight chassis 38 by inserting and engaging protrusions 44a, which are provided beneath the member 44, into and with engaging holes 45 which are punched so as to be formed both in the reflector 37 and the bottom portion 38b, as shown in a detailed drawing in a circle in FIG. 5.
The above-mentioned frame 34 is secured to the support surfaces 38e and 38f of the backlight chassis 38 while interposing the optical sheets 35 and the reflector 37 therebetween. In this case, as shown in FIG. 6, protrusion portions 38g having a square shape are formed at the four corners of the support surfaces 38e and 38f, and concave portions 35a are formed as a notch at the four corners of the optical sheets 35. By fitting the protrusion portions 38g into the concave portions 35a, the optical sheets 35 are positioned on the support surfaces 38e and 38f. 
Incidentally, as a prior art literature relating to the present invention, Japanese Patent Application Unexamined Publication No. Hei 11-306835 is cited.
In the above-described backlight unit 33, a thermal factor of the tubular lamp 36 that is the light source contributes to thermal expansion or thermal contraction of structural components thereof, and coefficients of thermal expansion and thermal contraction vary among the structural components. Therefore, friction develops at the time of thermal expansion or thermal contraction especially between the optical sheets 35, and the support surfaces 38e and 38f of the backlight chassis 38 and an interposition surface 34a of the frame 34 interposing the optical sheets 35 therebetween, which causes a problem of making a creaking sound.
In addition, as shown in FIG. 6, in a case where a gap H between the protrusion portion 38g of the backlight chassis 38 and the concave portion 35a of the optical sheets 35 which are used for positioning is set as clearance in consideration of a thermal expansion increment of the optical sheets 35 in use, the gap is too large at ambient temperatures during assembly of the backlight unit 33, so that positioning accuracy is not achieved. In contrast, in a case where the gap H is set in consideration of the positioning accuracy and without consideration of the gap H as clearance of the thermal expansion increment, the optical sheets 35 bend deeply because of no clearance, causing a creaking sound as described above or developing luminance irregularity.